Metal hydroxides and basic metal carbonates are usually prepared by precipitation from corresponding aqueous metal salt solutions by reaction with alkali hydroxides and alkali hydrogen carbonates, respectively. In that reaction, stoichiometric amounts of neutral salts are formed, which must be worked up or disposed of.
In order to avoid the formation of neutral salts, it has therefore been proposed according to U.S. Pat. No. 5,391,265 to prepare nickel hydroxide by the production of nickel ions by anodic dissolution and hydroxyl ions by the electrolytic decomposition of water, hydrogen being formed at the cathode in addition to precipitated nickel hydroxide. In that process, the electrolytic cell is charged with a conducting salt solution (sodium chloride and sodium sulfate), the conducting salt solution being fed back into the electrolytic cell again after separation of the precipitated nickel hydroxide. Accordingly, the process takes place substantially without the formation of neutral salts. A disadvantage of that process is that the nickel hydroxide is obtained in very finely divided form as a filterable but gel-like product having high bonded water contents, which product must subsequently be conditioned. The achievable particle size can be influenced only with great difficulty.
According to EP-A 684 324 it has been proposed to circulate separate anolyte and catholyte circuits in a two-chamber electrolytic cell divided by an anionic ion-exchange membrane, wherein nickel is dissolved anodically in the anode chamber, the anolyte contains ammonia as complexing agent, hydroxyl ions are produced in the cathode chamber and conveyed through the membrane into the anode chamber, the nickel ammine complexes are hydrolysed in the anolyte by means of an increase in temperature, and nickel hydroxide is precipitated and separated from the anolyte. The process allows the particle size of the nickel hydroxide to be controlled within wide ranges by controlling the hydrolysis process. However, the process is cost-intensive and susceptible to failure owing to the still inadequate useful life of commercially available membranes.
The object of the invention is to provide a process for the preparation of metal hydroxides that does not have the mentioned disadvantages. The process according to the invention also permits the preparation of basic metal carbonates substantially without the formation of neutral salts.
It has now been found that metal hydroxides or basic metal carbonates can be prepared in a two-step process, as follows: in a first step, a metal salt solution is obtained, using an alkali salt solution, by anodic dissolution of the metal, and an alkaline alkali salt solution is obtained by cathodic evolution of hydrogen, which solutions are combined in a second step in order to precipitate the metal hydroxide. The alkali metal salt solution obtained after separation of the metal hydroxide precipitation product is fed back into the electrolytic cell. That is made possible by the use of a three-chamber electrolytic cell in which the chambers are separated by porous membranes, by introducing an alkali salt solution into the intermediate chamber between the cathode chamber and the anode chamber. Basic carbonates are obtained by additionally introducing carbon dioxide into the cathode chamber or into the precipitation reactor of the second step.
Accordingly, the present invention provides a process for the preparation of metal hydroxides or basic metal carbonates by anodic dissolution of corresponding metals and precipitation of the hydroxides or basic carbonates in an aqueous medium, which process is characterised in that the anodic dissolution of the metal component takes place in the anode chamber of a three-chamber electrolytic cell, an aqueous auxiliary salt solution is fed continuously to the intermediate chamber arranged between the anode chamber and the cathode chamber and separated therefrom by porous membranes, an at least non-alkaline metal salt solution is removed continuously from the anode chamber, an alkaline auxiliary salt solution is removed continuously from the cathode chamber, and the at least non-alkaline metal salt solution and the alkaline auxiliary salt solution are combined outside the electrolytic cell in order to precipitate metal hydroxides or basic metal carbonates.
During the precipitation from the combined solutions there may optionally be supplied an alkali hydroxide solution for adjusting the desired precipitation pH value and a solution containing a complexing agent, for example an NH3 solution, for producing spherical precipitated products.
Basic metal carbonates are obtained in a simple manner by introducing carbon dioxide either into the cathode chamber or into the combined precipitation solution.
Suitable metals are those which form soluble salts in an aqueous medium, can be precipitated in a neutral or alkaline medium in the form of hydroxides and/or basic carbonates and which, when connected as the anode in the electrolytic cell, do not form non-conductive surface layers (oxides). The metals particularly preferably used are Fe, Co, Ni, Cu, In, Mn, Sn, Zn, Cd and/or Al. Nickel or cobalt anodes are preferably used.
Suitable auxiliary salts for introduction into the intermediate chamber of the electrolytic cell are chlorides, nitrates, sulfates, acetates and/or formates of alkali and/or alkaline earth metals. Sodium chloride and sodium sulfate are preferred. The auxiliary salt solution preferably has a concentration of from 1 to 3 mol/l.
The auxiliary salt solution introduced into the intermediate chamber flows through the porous membranes to the anode chamber and to the cathode chamber, whereupon, as a result of the effect of the electric field, partial ion separation of the auxiliary salt solution takes place into a component having excess anions, which flows to the anode, and a component having excess cations, which flows to the cathode. The auxiliary salt solution is preferably introduced into the intermediate chamber under a pressure such that the rate of flow through the porous membranes is greater than the migration rate of the anodically produced metal ions and the cathodically produced OH− ions in their respective solutions, so that the anodically produced metal ions and the cathodically produced OH− ions cannot pass into the intermediate chamber. On the other hand, the separation of the auxiliary salt solution into components having excess anions and excess cations is better, that is to say the transfer of neutral auxiliary salt into the anode and cathode chambers is lower, the lower the rate of flow of the auxiliary salt solution through the membranes. Optimum conditions can be determined by means of simple preliminary tests in dependence on the structural properties of the separation medium or its permeability or flow resistance. With regard to the separation effect and the electrical energy to be applied, it is possible to establish an optimum that is determined by the nature and concentration of the electrolyte. The rate of influx of the electrolyte must be so chosen that the ions having the higher mobility are at all events prevented from passing into the middle chamber. Preferably, the ratio of anions to cations in the auxiliary salt solution that passes through the membrane to the anode side is approximately 1.5 to 3 and, conversely, the ratio of cations to anions in the auxiliary salt solution that passes through the membrane to the cathode chamber is approximately 1.2 to 3.
The whole of the auxiliary salt solution introduced into the intermediate chamber preferably passes through the porous membranes.
Suitable membranes are porous, preferably woven, cloths or nets consisting of materials that are resistant to the auxiliary salt solutions, the anolytes and the catholytes. For example, polypropylene cloths such as are supplied by SCAPA FILTRATION GmbH under the name Propex may be used. Suitable cloths preferably have a pore radius of from 10 to 30 μm. The porosity may be from 20 to 50%.
The auxiliary salt solution having excess anions that passes into the anode space from the middle chamber is substantially neutralised by the anodic dissolution of the metal anode and continuously drawn off as anolyte. In order to avoid the formation of precipitated products in the anode chamber solution (anolyte), a small amount of acid may be fed into the anode chamber, preferably by feeding in an acid that contains the anion of the auxiliary salt solution. The anolyte discharged from the anode chamber preferably has a metal salt content of from 0.5 to 2 mol/l. At the cathode, hydrogen and OH− ions are formed according to the excess of cations in the auxiliary salt that have passed through the membrane to the cathode space. An alkaline auxiliary salt solution (catholyte) therefore flows over from the cathode chamber.
The anolyte and catholyte are subsequently subjected to a precipitation reaction in a precipitation reactor. A hydroxide solution may optionally be added in order to adjust the precipitation pH value, and complexing agents such as ammonia may optionally be added in order to achieve a spherical form of the precipitated products. For the preparation of basic carbonates, carbon dioxide is introduced into the catholyte or directly into the precipitation reactor. After separation of the precipitated product, an optionally alkaline auxiliary salt solution remains which, after being neutralised, is preferably fed back into the intermediate chamber of the electrolysis. It is also possible to store the anolyte and catholyte in intermediate containers and to carry out the precipitation discontinuously.
For the preparation of doped metal hydroxides, corresponding metal salt solutions of salts of the doping metals may be introduced into the precipitation reactor, in which case the amount of alkali hydroxide fed to the precipitation reactor for adjusting the precipitation pH value increases in a molar manner according to the amount of doping salts. A corresponding excess amount of neutral salts is therefore formed, which cannot be fed back into the intermediate chamber of the electrolytic cell.
Accordingly, for the preparation of mixed metal hydroxides it is advantageous either to use anodes that are alloyed according to the composition of the mixed metal hydroxide, or to provide in the anode chamber a plurality of anodes of the alloy metals, those anodes being subjected to electrolysis stream intensities the ratio of which corresponds to the (equivalent) ratio of the metals of the mixed metal hydroxide composition or, alternatively, to prepare the respective metal salt components separately in separate three-chamber electrolytic cells.
The precipitation reaction may also be controlled by the presence of complexing agents, for example ammonia, in the precipitation reactor. For example, in the preparation of nickel hydroxide, spherical nickel hydroxides are obtained by introducing ammonioma into the precipitation reactor.
Amphoteric doping metals, such as, for example, aluminium, may be introduced into the catholyte in the form of the aluminium salt or aluminates.
Following the precipitation, the precipitated product is separated from the combined auxiliary salt solution (mother liquor). That may be effected by sedimentation, by means of cyclones, by centrifugation or filtration. The separation may be carried out stepwise, the precipitated product being obtained in fractionated form according to particle size. It may also be advantageous to feed a portion of the mother liquor containing the small metal hydroxide particles as crystal nuclei back into the precipitation reactor once the large metal hydroxide particles have been separated off.
The mother liquor freed of the precipitated product, optionally after being worked up, is fed back into the intermediate chamber of the three-chamber electrolytic cell.
Working up serves to remove residual metal ions, to prevent impurities from becoming concentrated, and to re-adjust the concentration and composition of the auxiliary salt solution, for example to strip any complexing agent optionally introduced for precipitation purposes. Working up of the mother liquor may take place in a split stream.
On the other hand, the process is insensitive as regards working up of the auxiliary salt solution. Accordingly, it is generally harmless if the complexing agent is fed back into the intermediate chamber with the mother liquor. Likewise, the process is scarcely impaired by the introduction of small amounts of metal ions into the intermediate chamber. The metal ions are precipitated in the intermediate chamber or in the catholyte as hydroxide slurry that may settle out, or are discharged into the precipitation reactor with the catholyte as very finely divided hydroxide.
With the process according to the invention there is made available an extremely flexible electrolytic process for the preparation of metal hydroxides, in which substantially no further materials are required other than the materials of the anode metal and water as well as small amounts of acids and/or bases for regulating the pH value, and accordingly no secondary products are formed either. The flexibility is the result of the electrolytic separation of a recirculable, neutral auxiliary salt solution into an acid and an alkaline component as it passes through robust, porous, electrochemically inactive membranes. It is thus possible to discharge the metal ions and the hydroxide ions from the electrolytic cell in the form of separate solutions and combine them again only for the purposes of precipitation. As a result, the precipitation itself can be controlled independently without affecting or being affected by the electrolysis process.
Accordingly, with the process according to the invention there is made available an extremely flexible process for the preparation of metal hydroxides or basic carbonates.
The person skilled in the art is readily capable of carrying out further variations adapted to the particular requirements for the preparation of a specific product. For example, it is possible, accepting slightly higher pressures in the intermediate chamber, to render the conducting salt anion/cation ratio that passes to the anolyte or catholyte more advantageous, by using multi-layer filter cloths. It is also possible to separate the middle chamber on the cathode side and on the anode side by different separation media (filter cloths, diaphragms, etc.) in order to permit different flow conditions (rates) into the cathode space and the anode space. Furthermore, while maintaining the three-chamber principle, that is to say the separation of the anode space and the cathode space by a middle chamber, arrangements of the electrodes and separation media that are completely different in terms of geometry are possible. For example, the electrodes may be arranged concentrically as in a tubular condenser. In the middle of a cylindrical cell there is a cylindrical electrode, the counter electrode is arranged concentrically to that central electrode as a tube. In the tubular space between the two electrodes is the middle chamber, which is likewise arranged concentrically and which is formed by two tubular filter cloths, diaphragms or similar separation media extending parallel to each other.
The invention also provides a device for the preparation of metal hydroxides, containing a three-chamber electrolytic cell, a precipitation reactor and means of separating solids from the product discharged from the precipitation reactor, the electrolytic cell being divided by means of porous membranes into an anode chamber, an intermediate chamber and a cathode chamber, having an inlet to the intermediate chamber, an outlet from the anode chamber and an outlet from the cathode chamber, an inlet of the precipitation reactor being connected to the outlet from the anode chamber, and a further inlet of the precipitation chamber being connected to the outlet from the cathode chamber.
The cathode chamber also has an outlet for cathodically produced hydrogen. There may also be provided means of introducing subordinate amounts of auxiliary reagents such as acid into the anode chamber, base into the precipitation reactor, both acid and base for adjusting the pH value, as well as complexing agents and doping agents into the precipitation reactor.
In order to adjust a particular pH value, acid may be fed into the anode chamber via pipe 16.
Furthermore, carbon dioxide may be introduced via pipe 17 in order to prepare basic metal carbonates.
The products 41 and 42 discharged from the electrolytic cell 1 are fed into the precipitation reactor 2. The precipitation reactor contains, for example, a high-speed stirrer 21. The precipitation reactor may also be in the form of a loop-type or propulsive-jet reactor or in a different form. The precipitated suspension flows over from the precipitation reactor in pipe 43. It is also possible to provide inlet devices 22, 23 and 24 for the introduction of auxiliary and modifying agents, such as for adjusting the pH value, doping and/or influencing the precipitation by introduction of complexing agents, or for the introduction of CO2 for the preparation of basic carbonates. Depending on the desired precipitation conditions, the precipitation reactor 2 may also be in the form of a reactor cascade, partial streams of the products 41 or 42 discharged from the electrolytic cell being introduced into the individual reactors of the cascade.
The precipitated suspension passes via pipe 43 into the separating device 3, which in this case is shown as a hydro-cyclone, from which the precipitated solid is largely drawn off via the bottom outlet 31 and the precipitation mother liquor freed of solid flows over via pipe 44 to the means for working up 45. Arrow 48 shows diagrammatically the introduction of working-up reagents and the removal of interfering components that may be present. The worked-up mother liquor can be fed back into the intermediate chamber I via pipe 47 and pump 46.